In a manufacturing process of semiconductor devices, a plasma etching is performed on a silicon-containing dielectric layer (for instance, a SiO2 film, a SiOC film, or the like) by using an organic layer such as a photoresist as a mask, to thereby form, for example, contact holes. In general, a gaseous mixture of, for example, C4F6/Ar/O2 is employed as a processing gas for plasma generation in the plasma etching. (see, for example, Patent Reference 1).
Further, there has been proposed using a processing gas containing a fluorocarbon gas at least having four carbon atoms with a F/C ratio equal to or smaller than a value of 2; an argon or xenon gas; and an oxygen gas to perform a plasma etching of SiO2 selectively against a photoresist (see, for example, Patent Reference 2).    [Patent Reference 1]
Japanese Patent Laid-open Application No. 2002-231596    [Patent Reference 2]
Japanese Patent Laid-open Application No. 2004-512668
When forming contact holes of high aspect ratios by using an organic layer such as a photoresist as a mask, a high selectivity of a target layer to be etched against the mask is required. The necessity to meet such a requirement is getting stronger in order to achieve a thickness reduction of a mask layer, while improving productivity. Further, there is an increasing demand for achieving a high etching rate as well as the demand for maintaining the selectivity high. Thus, a plasma etching method capable of satisfying both the high selectivity and high etching rate is required to be developed.